1. Field of the Invention
The invention relates to a system and method for location determination, particularly to a system and method for location determination using time differences.
2. Description of the Related Art
The conventional system for location determination can be classified into three types: proximity, triangulation, and scene analysis. With regard to the proximity type, referring to U.S. Pat. No. 5,493,283, the precision of location determination is corresponding to the transmission range of bases and sensors. The higher the precision is, the smaller the transmission range must be. Therefore, more bases or sensors are needed, and the installation cost is higher.
Referring to U.S. Pat. Nos. 5,646,632 and 6,275,705, the triangulation type is the most popular one. The triangulation type mainly uses the distances or the angles between bases and sensors to determine the location. If using the distances for determination, three distances between three reference points and the user are needed to determine the location of the user on a plane. The distances can be obtained according to propagation time of signals or according to the decay of signal intensities. If using the angles for determination, it is necessary to know two angles formed by the user and two reference points so as to determine the location. The angles may be obtained by using some specially designed hardware.
Scene analysis uses the information of scenes or objects, for example, the signal intensity of the location compared with that in the database, to determine the location. Referring to U.S. Pat. No. 6,839,027, it is necessary to build a database for every environment so that the setup is more complicated. In the method of using multiple signal intensities of bases in the environment, signal intensities of each location in the environment must be measured beforehand in order to build the database of signal intensities. And then the probable location can be determined by utilizing the present measured signal intensities.
Referring to U.S. Pat. No. 6,865,347, it discloses a photography method by utilizing devices or some specially designed hardware to measure the direction of incident light, the angle of elevation, and the distance. However, the method is easily influenced by sunlight and other light sources so as to decrease the practicability of the method.
Referring to FIG. 4, it shows a conventional radar location determination system. The radar location determination system 40 comprises a radar 41 and a detected object 42. The radar location determination system 40 uses the time of reflection of microwave to calculate the distance, and also determines whether the detected object 42 is a friendly plane or an enemy plane according to the ability for responding some predefined codes. However, the radar location determination system 40 uses single directional radar 41 to transmit and receive signals, and the single directional radar 41 is also used to determine the direction and the elevation angle of the signal. Therefore, the radar location determination system 40 cannot rotate the single directional radar 41 by a large angle within the period before the reflecting microwave from the detected object 42 reaches the radar location determination system 40, in order to receive the responding signals and to determine the location precisely. Thus, after the radar 41 transmits a detecting signal, the radar location determination system 40 has to be static or rotate slowly within the period for waiting for receiving the reflecting signal from the detected object 42 so as to calculate the distance of the detected object 42. Therefore, the radar location determination system 40 has longer detecting time.
Consequently, there is an existing need for a system for location determination to solve the above-mentioned problem.